The electrochemical CO2 reduction reaction (CO2RR) is a subject of considerable current interest that is motivated by the desire to develop methods for converting atmospheric CO2 to fuels using electrical energy generated from renewable sources such as wind and solar power. However, commercial implementation of the CO2RR has yet to be realized, primarily due to challenges associated with electrocatalyst activity and selectivity. The reaction requires approximately 800 mV of overpotential to produce hydrocarbons and alcohols (Hori, Y., Takahashi, R., Yoshinami, Y. & Murata, A. Electrochemical Reduction of CO at a Copper Electrode. J. Phys. Chem. C 101, 7075-7081 (1997); Durand, W. J., Peterson, A. A., Studt, F., Abild-Pedersen, F. & Nørskov, J. K. Structure effects on the energetics of the electrochemical reduction of CO2 by copper surfaces. Surf Sci. 605, 1354-1359 (2011); Li, C. W. & Kanan, M. W. CO2 reduction at low overpotential on Cu electrodes resulting from the reduction of thick Cu2O films. J. Am. Chem. Soc. 134, 7231-4 (2012); Peterson, A. A., Abild-Pedersen, F., Studt, F., Rossmeisl, J. & Nørskov, J. K. How copper catalyzes the electroreduction of carbon dioxide into hydrocarbon fuels. Energy Environ. Sci. 3, 1311 (2010); Tang, W. et al. The importance of surface morphology in controlling the selectivity of polycrystalline copper for CO2 electroreduction. Phys. Chem. Chem. Phys. 14, 76-81 (2012)), resulting in an overall energy conversion efficiency for the cathodic process of roughly 45%. Moreover, the reaction can produce up to 16 different products depending on the composition of the electrocatalyst and the applied potential (Kuhl, K. P., Cave, E. R., Abram, D. N. & Jaramillo, T. F. New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy Environ. Sci. 5, 7050-7059 (2012)). This lack of selectivity results in additional processing costs associated with product separations, which further reduce the cost-effectiveness of the technology. For these reasons there is a great deal of interest in the discovery and development of catalysts for the selective formation of potential fuels by the reduction of CO2. Since the reaction produces a mixture of gaseous and liquid phase products, a combination of analytical techniques must be employed to fully characterize electrocatalyst activity and selectivity at a given potential. Gaseous products are typically analyzed via gas chromatography by sampling the electrochemical cell headspace in-situ while liquid phase products are analyzed using either liquid chromatography or nuclear magnetic resonance ex-situ (Kuhl, et al. (2012); Hori, Y., Murata, A. & Takahashi, R. Formation of Hydrocarbons in the Electrochemical Reduction of Carbon Dioxide at a Copper Electrode in Aqueous Solution. J. Chem. Soc. Faraday Trans. 1 85, 2309-2326 (1989)). However, because many of the minor products are produced with Faradaic efficiencies of less than 5%, constant potential electrolysis must be carried out for roughly one hour in order to reach the analytical detection limits of these techniques. As a consequence, there is considerable interest in the development and implementation of an analytical technique that can be used for on-line characterization of the products of the CO2RR
Differential electrochemical mass spectrometry (DEMS) is an analytical technique that combines an electrochemical half-cell experiment with mass spectrometry, uniting the two with a pervaporation membrane (Wolter, O. & Heitbaum, J. Differential Electrochemical Mass Spectroscopy (DEMS)-. Berichte der Bunsengesellschaft für Phys. Chemie 6, 2-6 (1984)). Because the analysis time of mass spectrometry is on the order of a second, the formation of gaseous or volatile reaction products can be monitored continuously in-situ. By relating the relevant mass ion currents to the Faradaic efficiencies of the various reaction products, the activity and selectivity of a given electrocatalyst can be studied in real time as a function of the applied potential. This ultimately enables the potential dependence and transient nature of the reaction selectivity to be rapidly screened.
The efficacy of DEMS is heavily reliant on the design of the electrochemical cell, which must be capable of achieving both a rapid response time and a high product collection efficiency (Baltruschat, H. Differential electrochemical mass spectrometry. J. Am. Soc. Mass Spectrom. 15, 1693-706 (2004)). As a result, electrochemical cells are specially designed for this application. There are also additional design criteria that need to be met. The working and counter electrodes should be parallel in order to ensure that the current density does not vary as a function of position on the electrode surface. If this design criterion is not met then the partial current of a given product will vary across the electrode surface, making it impossible to make accurate conclusions about the reaction selectivity at a given potential. Another design constraint is that the electrodes should be spatially separated by either a proton or anion-conducting membrane. If the electrodes are not spatially separated, it is possible that Faradaic current from oxygen reduction can occur while studying CO2RR electrocatalysts. The amount of Faradaic current from this unwanted reaction cannot be quantified since the only product is water. In the absence of spatial separation of the electrodes, products formed at the cathode can undergo oxidation at the anode thereby reducing the concentration of products that can be detected. Yet another design constraint is that the cross sectional area of electrolyte between the working and counter electrodes should be large. This will result in a low cell impedance and reduce the propensity of bubble formation to break the electrical continuity of the electrolyte between the electrodes. Since the CO2RR can become diffusion limited in stagnant electrolytes in less than three minutes at 10 mA/cm2, electrolyte convection must be used to study a given electrocatalysts true activity and selectivity for this reaction. As a result, the volume of electrolyte between the working electrode and the pervaporation membrane should be small so that the delay time between product generation and detection can be minimized. If the cell volume is allowed to be excessively large then the electrolyte flow rate will also have to be excessively large in order to achieve acceptable delay times. This is an issue because excessively high electrolyte flow rates will result in product dilution and reduced detectability. Finally, the surface area of the working electrode should be large so that the concentration of reaction products can be maximized.
DEMS cell designs described in the literature suffer from a variety of drawbacks including non-parallel electrode configurations, high impedances, and reactant diffusion limitations. The first DEMS cell to use electrolyte flow was the thin-layer flow cell (Hartung, T. & Baltruschat, H. Differential Electrochemical Mass Spectrometry Using Smooth Electrodes: Adsorption and H/D-Exchange Reactions of Benzene on Pt. Langmuir 6, 953-957 (1990)). This cell geometry consisted of a thin layer of electrolyte, approximately 100 microns thick, that separated the working electrode and the pervaporation membrane. This cell geometry suffers from a low product collection efficiency because products generated near the electrolyte outlet are swept out of the chamber before mass transport to the pervaporation membrane can occur. Furthermore, the non-parallel electrode configuration makes it impossible to study the reaction selectivity. To solve the issues of low product collection efficiency the dual thin-layer flow cell was developed (Jusys, Z., Kaiser, J. & Behm, R. J. A novel dual thin-layer flow cell double-disk electrode design for kinetic studies on supported catalysts under controlled mass-transport conditions. Electrochim. Acta 49, 1297-1305 (2004)). This cell geometry locates the working electrode and the pervaporation membrane in separate chambers, connected by four transfer capillaries. While this cell geometry achieves higher collection efficiencies, it suffers from a non-parallel electrode configuration and restrictive pathways for ion conduction. A modified version of the DEMS instrument has also been developed recently that enables online product analysis from a traditional H-cell (Wonders, A. H., Housmans, T. H. M., Rosca, V. & Koper, M. T. M. On-line mass spectrometry system for measurements at single-crystal electrodes in hanging meniscus configuration. J. Appl. Electrochem. 36, 1215-1221 (2006)). This approach employs a capillary tube with a porous Teflon tip, which is brought within 20 microns of the electrode surface in order to sample products formed by the CO2RR. While this geometry enables the use of a parallel electrode configuration it also makes product quantification impossible because the collection efficiency is extremely low and highly dependent on the precise orientation of the porous Teflon tip with respect to the electrode surface. Furthermore, the trends mass spectrometer ion currents do not properly reflect the trends in the partial currents of the different reaction products because of mass transport limitations caused by the close proximity of the sampling tip to the surface of the working electrode.